After Perihelion, 3I/ATLAS Turns Green — Avi Loeb Explains the Latest Observations
- ACIMA WORLD NEWS
- Dec 16, 2025
- 3 min read
The interstellar object 3I/ATLAS has entered a new observational phase after passing perihelion. Recent images reveal that the glowing halo surrounding the object has shifted in color — from red to green — prompting renewed scientific interest.
In a recent article, Harvard astrophysicist Avi Loeb reviewed these observations and explained what the color change may reveal about the physical processes taking place around 3I/ATLAS.
Rather than speculation, his analysis focuses on measurable data from ground-based and space-based telescopes.
A Post-Perihelion Image of 3I/ATLAS from Gemini North
On November 26, 2025, exactly four weeks after perihelion, 3I/ATLAS was imaged by the Gemini Multi-Object Spectrograph (GMOS) on the 8.1-meter Gemini North telescope at Maunakea, Hawaii.
The composite image combines exposures taken through four filters — blue, green, orange, and red — with peak brightness centered on the nucleus. The most striking feature is a green glowing halo, accompanied by a distinct sunward-pointing anti-tail.
Loeb notes that the green emission is likely produced by diatomic carbon (C₂) molecules, which are known to emit green light and are commonly observed in cometary comae.
From Red to Green: What Changed?
Before perihelion, observations told a different story. On September 4, 2025, images taken by the Gemini South telescope in Chile showed a predominantly red glow surrounding 3I/ATLAS.
The transition from red to green after perihelion suggests that the molecular composition of the gas plume released by the object changed as it passed close to the Sun. Increased solar heating can alter which molecules are released or excited, leading to observable shifts in color.
According to Loeb, this behavior indicates that 3I/ATLAS is not merely a passive collection of dust, but an active object responding chemically to solar radiation.
X-Ray Emission Confirms Ongoing Activity
Independent confirmation of this activity comes from X-ray observations.
On December 3, 2025, ESA’s XMM-Newton X-ray Observatory observed 3I/ATLAS for approximately 20 hours from a distance of about 284 million kilometers. The detected X-ray glow is consistent with interactions between the solar wind and a surrounding cloud of gas.
These findings align closely with earlier observations by JAXA’s XRISM mission, reinforcing the conclusion that a substantial gas plume has surrounded 3I/ATLAS for at least five months.
An Active Interstellar Object
Taken together, multiple lines of evidence now point to a more complete physical picture of 3I/ATLAS:
A color change from red to green after perihelion
Sustained gas release over several months
X-ray emission produced by solar wind interactions
Loeb emphasizes that these observations paint a picture of a dynamic, comet-like interstellar object, rather than a dormant body drifting passively through the Solar System.
A Data-Driven Interpretation
Throughout his analysis, Loeb avoids dramatic claims. Instead, he highlights how each observation fits within known physical mechanisms — and where additional data may refine or revise current understanding.
As observations continue and new datasets are analyzed, 3I/ATLAS is offering researchers a rare opportunity: a close look at how material formed around another star responds when exposed to our Sun.
Comments Welcome
What do you make of the green emission observed after perihelion? We welcome your thoughts and interpretations in the comments.
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